With the backlog in its cable replacement programme proving difficult to clear, Nebraska Public Power District (NPPD) decided to evaluate a new strategy based on testing its cable network with Megger equipment.
This approach made it possible for NPPD to quickly clear the backlog by concentrating budget and resources on only those cables where the risk of imminent failure was high.
Until recently, NPPD operated a cable replacement programme based on two criteria. The first of these criteria was faults – if more than three faults occurred on a single cable section, it was deemed in need of replacement. NPPD’s second criterion for cable replacement was age, with the oldest cables, many of which have been in service for more than 25 years, scheduled for replacement first.
The overall result was that NPPD had a backlog of cables identified within the cable replacement programme. However, replacing the older cables on the basis of their age alone meant cables that still had a useful working life were being taken out of service unnecessarily.
To address these issues, NPPD decided to investigate the possibility of setting up a more efficient and cost-effective cable replacement programme based on routine testing to assess the true condition of its cable network. NPPD considered both partial discharge (PD) and Tan Delta (TD) testing, and decided that the latter would be more straightforward to apply in the field, and that the results would be easier to interpret.
To carry out the test, NPPD chose a Megger VLF45 Sine Wave 45 kV test set with integrated Tan Delta facilities. The main factors that influenced this choice included the compact construction of the VLF45, its Tan Delta functionality, its ability to test up 35 kV class cables, and the excellent technical support received from Megger.
NPPD purchased one instrument and has now used it extensively to investigate the condition of its state-wide cable network. NPPD personnel found it to be intuitive in operation, with a simple test set up and results that are easy to analyse and interpret. Those results have proved invaluable, as they have revealed that 70% of the cables NPPD tested need no further action. This has allowed the utility to concentrate its resources on the cables that are most likely to give problems.
By moving to a cable replacement programme based on actual test results produced by a Megger VLF45 Sine Wave 45 kV test set rather than relying on arbitrary age and condition criteria, NPPD has now eliminated its cable replacement backlog. Additional benefits are that it has been able to improve its System Average Interruption Duration Index (SAIDI) and System Average Interruption Frequency Index (SAIFI) ratings, both of which are important and widely monitored indicators of a power utility’s performance.
Cable faults are a major challenge for all power utilities, especially in parts of the world where much of the supply network is approaching, or in some cases has already exceeded, its original design lifetime. Wholesale replacement of aged cables to address this issue is rarely an option, so a rational and well-structured replacement programme is essential.